Multi-layer coating systems have been utilized to coat automobiles for a number of years but, during the early years of the development of these systems, the solvents were organic. As environmental regulations have become more stringent, and the costs of organic solvents have risen, organic solvent-borne coating systems have become less desirable. Recent research efforts in the field of coating compositions for automotive multi-layer coatings have thus focused on the development of water-borne systems.
The shift from organic solvents to water for dispersing and applying resins in multi-layer coating systems has solved many of the environmental and cost problems typically associated with the use of organic solvents, yet water-borne coating systems are not without problems of their own.
The application of a multi-layer coating to a metal substrate such as an automobile body is greatly facilitated by coating systems which have quick drying characteristics, both during and after application of each coating layer. For example, a quick drying coating system permits application over a broad range of ambient humidities while minimizing problems of flow and sag. The resulting lack of any need to carefully control humidity in the spray zone lowers energy costs. Moreover, a quickdrying system shortens the waiting time required between coating layers and lowers the temperature required for any flash drying steps which may be required between the steps of applying multiple coating layers. In early coating systems, these advantages were achieved by the use of low boiling organic solvents. However, with the shift to water-borne coating systems, the more difficult drying of water from a coating film has presented problems. Because of the tendency of water to hydrogen-bond to polar functional groups in the coating composition and the higher heat capacity and boiling point of water, higher temperatures and more heat energy are required to remove the solvent from the coating during drying and curing steps. This means that longer drying times are required between the application of successive coating layers in a multi-layer coating system or, alternatively, flash drying steps of high temperature or longer duration between such applications of successive layers. In so-called "wet-on-wet" multi-coat systems in which two or more coatings are applied to the substrate without intermediate curing of each coat, water which remains in underlying layers after the application of overlying layers tends to boil out during the baking or curing step. This solvent popping disrupts the overlying layer(s) and results in an undesirable rough, uneven surface to the finished coating.
Because of the ease with which acrylic-based polymers can be dispersed in water, early research efforts in water-borne coatings focused on these polymers as the resins of choice in water-borne systems. However, coating compositions based on acrylic polymers have a greater tendency to trap and retain water. Moreover, because of their tendency to retain water, water-borne acrylic coating systems generally must be applied within a narrow range of ambient humidities in order to prevent sagging or flowing of the film during application. For example, U.S. Pat. No. 4,007,306 discloses a method of applying an aqueous all-acrylic coating composition to metal substrates which is limited to ambient humidities ranging between about 40% and 60%.
To overcome the aforementioned disadvantages of water-borne acrylic systems, research efforts have turned toward the development of water-borne coating compositions which are based upon resins other than acrylics. U.S. Pat. Nos. 4,794,147 and 4,791,168, for example, disclose water-borne coating systems based upon all-polyurethane chemistry.
Non-acrylic water-borne coating systems are attractive because they can be formulated to possess a degree of hydrophobicity which overcomes the water-retaining disadvantages characteristic of acrylic water-borne systems. The resulting coating compositions can thus be applied over a wider range of ambient humidity without sagging. However, the optimization of this property of non-acrylic water-borne coating systems often leads to resins which do not adequately wet the surface of pigmenting agents which are incorporated into the coating compositions, frequently leading to coating compositions which have lower shelf life and/or color stability. In general, increasing the hydrophobicity of the coating system to reduce interaction with the aqueous solvent tends to simultaneously reduce the ability of the resin to interact with and wet the surface of highly polar pigments. In such coating systems, this problem can be overcome, but usually requires burdensome or time consuming pigment grinding conditions to form pigment pastes. Even when such precautions are taken in preparing pigment pastes in many non-acrylic systems, problems are often observed relating to pigment particle agglomeration, and shortening of the effective shelf life of the pigment paste and/or coating composition.
It is therefore an object of the present invention to provide an aqueous based or water-borne coating composition for metal and/or plastic substrates which overcomes the various disadvantages of prior art all-acrylic or all-polyurethane water-borne coating systems and which provides a coating which can be applied over a wide range of ambient humidities, and which has good pigment wetting and dispersion characteristics with improved shelf life and color stability.